This invention relates to the sensing of the depth of a borehole as the borehole is being formed in a formation.
It is well known to use steerable drilling systems in the formation of boreholes to permit control over the drilling direction, and hence the direction in which the borehole is extended. By appropriate control over the steerable drilling system, the borehole can be drilled along substantially a predetermined, planned route.
When a curve or dog leg is formed in a borehole, the severity or sharpness of the curve is conventionally expressed in units of degrees/100 ft, and the steerable drilling systems used are designed to achieve deflection of the borehole away from it previous centreline as the borehole is extended.
In order to control the operation of such steerable drilling systems it is important to know the depth of the borehole, and to be able to transmit such depth information to the control system of the steerable drilling system. In the past, this has been done by continuously measuring the absolute hole depth at the surface and transmitting this information to the downhole components of the steerable drilling system. The regular or continuous transmission of hole depth information in this manner uses up a significant part of the available data transmission capacity and so is undesirable.